The present invention relates to an activated resinous composition useful for mounting of flip-chip or the like and a method for surface mounting using this activated resinous composition.
Conventionally, surface mounting technologies for various parts such as BGA parts have been carried out by a method comprising the steps of fluxing a printed wiring board; mounting BGA parts on the printed wiring board; reflow soldering; and cleaning and removing flux; filling space between the printed wiring board and the BGA parts with an underfill resin and hardening the underfill resin. The flux containing a compound having a carboxylic acid such as a rosin resin as an activator is well known (PATENT DOCUMENT 1, claim 2).
Recently, the number of chips mounted on the BGA part tends to increase for functional advancement and a body size of the BGA part correspondingly tends to grow in size.
However, having grown in size, the BAG part itself may interface with the desired cleaning effect and leave unremoved flux (flux residue) behind in the step of cleaning and removing flux. Consequently, there has been possibility that the activator ingredient contained in flux residue might cause corrosion reaction in the subsequent step of heat hardening of the underfill resin.
To overcome such problem, cleaning-free flux (flux free from requirement of cleaning) characterized in that the activator has active force sufficiently low to restrict possibility of corrosion has already been proposed (PATENT DOCUMENT 2). However, when such cleaning-free flux is used, this cleaning-free flux itself may generate cracked gas in the step of heat hardening the underfill resin and, in consequence, destroy the BGA part.
With the BGA part having grown in size, joints in the BGA part may interfere with a desired filling effect in the step of filling the underfill resin. Particularly when there are surface irregularities (e.g., circuit irregularities and/or solder mask irregularities) on the surface of the printed wiring board, it is often impossible to fill every hole and corner of the irregularities completely with the underfill resin and, as a consequence, voids and/or unfilled space may leave behind, significantly deteriorating quality and reliability of the product. Furthermore, if deficiency such as void is overlooked and the subsequent step of hardening the underfill resin, it will be no more possible to repair the product and such faulty product must be scrapped. This leads directly to reduction of yielding percentage.    [PATENT DOCUMENT 1] Japanese Patent Application Laid-Open Publication No. 2004-152936    [PATENT DOCUMENT 2] Japanese Patent Application Laid-Open Publication No. 2002-237676